Two highly specific polypeptide inhibitors of human Hageman factor (Factor XII) fragment(s) (HFf) were purified from corn seeds (Mr 12 000; pI 5.1, 6.3, and 7.7) and from pumpkin seeds (Mr 4 000; pI 8.3). They form 1:1 molar complexes with HFf and bovine trypsin, are "arginine" inhibitors, markedly prolong the activated partial thromboplastin time of human plasma, and do not inhibit human plasma or urinary kallikreins, plasmin, alpha-thrombin, hog pancreatic kallikrein, bovine Factor Xa, or alpha-chymotrypsin. Human plasma prokallikrein activated briefly by catalytic amounts of HFf gave a heavy chain of Mr 53 000 and two light chains of Mr 40 000 and 37 000, corresponding to the 88 000- and 85 000-dalton proenzyme forms. Further incubation after inhibiting HFf by excess corn inhibitor converted the Mr 53 000 chain into 33 000- and 20 000-dalton fragments.